The invention generally concerns an apparatus for resistance welding and more particularly an apparatus for resistance welding with different welding currents.
A typical form of apparatus for resistance welding includes a welding transformer with a primary side and a secondary side. Arranged on the secondary side are two welding electrodes which are connected to the welding transformer. The welding electrodes are also each arranged on a respective operating or positioning arm, of which at least one is movable by way of an actuator drive. Also arranged at the primary side is a regulating arrangement with electronically controllable switching members disposed between the welding transformer and the connection to a three-phase mains network. The regulating arrangement serves to regulate the voltage at the primary side.
Such an apparatus is intended for a resistance welding procedure in which the welding electrodes are pressed with a certain contact pressure force against both sides of the articles to be welded, for a certain period of time. For example plates or sheets can be welded together with that apparatus.
Conventional arrangements for producing a welding current in the case of ac resistance welding apparatuses are well known from the state of the art. In one such arrangement a voltage is tapped off between two connections of the three-phase network and fed to the welding transformer. Provided on the primary side of the welding transformer are two thyristors which are arranged parallel to each other with opposite forward directions and which form electronic switching members. The two connections can be connected to two phases of the three-phase network or to the neutral conductor and a phase of the network which usually involves a network frequency of 50 Hz or 60 Hz.
The above-discussed arrangement with the thyristors is inexpensive and generally reliable in operation. It will be noted however that the regulating options afforded with that arrangement are limited as the thyristors are switched only once in each half-wave of the current on the primary side and are switched off only at the zero-passage of the current on the primary side. Such an arrangement also suffers from further disadvantages which are generally known in this context and which therefore do not need to be further discussed herein.
The disadvantages of the above-discussed arrangement can be avoided by a resistance welding apparatus which has been offered in the past by the present patentees and which is operable to convert alternating current into direct current. That apparatus comprises a rectifier with a B-6 circuit, that is to say a bridge circuit with six diodes, with an inverting converter, also referred to as inverter, connected on the output side. The apparatus also has a smoothing capacitor. The voltage of all three phases, which is tapped off from the three-phase network, is rectified by the rectifier and chopped and inverted by the inverter. Depending on the capacity and operational efficiency of the inverter it generates a series of rectangular pulses of alternate polarity in the medium-frequency range, which can be referred to for the sake of brevity as the MF-range, at for example 1000 Hz, or in the high-frequency range, which can be referred to for the sake of brevity as the HF-range, at for example 20,000 Hz. In those apparatuses that voltage is applied to the primary side of an MF-transformer or HF-transformer designed for the corresponding frequency. The medium-frequency or high-frequency transformer voltage is rectified on the secondary side by high-current diodes and fed to the welding electrodes. The mode of operation of that apparatus is generally known and therefore does not need to be further discussed herein.
The essential parameters in regard to resistance welding are on the one hand the welding current and on the other hand the welding time for which the welding current flows. In the above-discussed apparatuses both parameters are controlled by a welding control system. A further parameter however which also has a crucial influence on resistance welding is the contact pressure force which has already been referred to hereinbefore. Hereinafter in this specification the contact pressure force will be referred to generally as the welding force.
In the above-discussed prior apparatuses the welding force is afforded for example by a pneumatic or hydraulic actuating drive for moving one of the control arms on which one of the welding electrodes is arranged, while the other control arm with the other welding electrode is arranged fixedly and rigidly. Instead of the pneumatic or hydraulic drive however it is also possible for the control arm to be moved by means of an electric motor. In comparison with the pneumatic or hydraulic drives, an electric motor drive is faster, more precise and can be better controlled. Furthermore, the wear of the individual components which make up the welding tongs assembly formed by the control arms and the welding electrodes, and in particular the wear suffered by the welding electrodes, is less by virtue of the controllable movement and the force settings which can be achieved in that case.
The electric motor in the above-discussed prior apparatuses is controlled by a drive control system which is provided exclusively for controlling the electric motor. In that case, the drive control system co-operates with the welding control system in such a way that matching of the above-indicated parameters in order to achieve a good welding result is possible only at the cost of a relatively high level of complication. In addition, the two separate control systems, more specifically the drive control system and the welding control system, means that the costs in terms of material for producing the apparatuses in question are also high.
An object of the present invention is to provide an apparatus for resistance welding by means of frequency inverter technology in which the drive control system for the drive is simplified so that the apparatus production costs are reduced.
Another object of the present invention is to provide an apparatus for resistance welding which is of a simplified structure generally and also in terms of its operating systems while nonetheless affording reliable operational results.
Still another object of the present invention is to provide a resistance welding apparatus capable of implementing welding procedures using different welding currents, which affords enhanced versatility of operation without the requirement for complicated operating and control systems.
Yet another object of the present invention is to provide a resistance welding apparatus which is so designed that components which are present in the apparatus can be used for more than just one operational purpose.
In accordance with the principles of the present invention the foregoing and other objects are attained by an apparatus for resistance welding, with different welding currents, comprising a welding transformer having a primary side and a secondary side, and first and second welding electrodes which are connected at the secondary side to the welding transformer and which are each arranged on a respective control or positioning arm. The apparatus has at least one drive for moving at least one of the control arms. A regulating and control arrangement which is connected on the primary side between the welding transformer and a connection to a three-phase network comprises a rectifier circuit connected to all three phases of the three-phase network and a controllable welding inverter which converts the rectified voltage into voltage pulses of alternating polarity at a predetermined frequency. At least one electronic unit of the rectifier circuit and/or the welding inverter is connected to the drive for controlling and/or powering the drive.
The at least one drive for moving at least one of the control arms may be more particularly an electric motor, and the movement of the control arms may be either a direct movement of the control arm or an indirect movement by means of an intermediate member. The controllable welding inverter converts the rectified voltage into voltage pulses of alternating polarity at a predetermined frequency of for example between 1000 Hz and 20,000 Hz.
It will be seen from the description hereinafter of preferred embodiments of the invention that the apparatus of the invention enjoys the advantage that it saves on a control unit, more specifically the separate drive control system of the above-discussed prior apparatuses, thus affording a reduction in the amount of material required to produce the apparatus and thus a reduction in the manufacturing costs generally. Electronic components which are required for producing and controlling the welding current and which are thus already provided in the apparatus are in part or completely also used for controlling and powering the drive. That affords a saving on important, major and expensive electronic components, in comparison with the prior apparatuses.
In accordance with a preferred feature of the invention at least one electronic component of the welding inverter is connected between the rectifier circuit and the drive. It is of particular advantage for the welding inverter per se, that is to say the entire welding inverter, to be connected between the rectifier circuit and the drive. The welding inverter which is then used jointly by the welding transformer and the drive in that case converts the rectified voltage both for the drive and also for the welding transformer.
In a further preferred feature of the invention not just individual electronic components of the rectifier circuit are connected to the drive, but the rectifier circuit per se is connected to the drive. That design configuration is suitable for a drive which is operated with dc voltage. If in contrast the apparatus uses a drive which requires ac voltage, then the welding inverter is preferably connected between the rectifier circuit and the drive in order to convert the rectified voltage into an ac voltage again. As an alternative thereto, it is possible to use two different inverters. In that case, on the one hand the above-specified welding inverter is connected between the rectifier circuit and the welding transformer, while on the other hand a further inverter, which can be referred to hereinafter generally as the drive inverter, is connected between the rectifier circuit and the drive. That can be particularly advantageous as, in terms of the electronic components of the drive inverter, it is possible to have recourse to using less expensive units than those of the welding inverter which, by virtue of being required to generate the high welding current, have to meet particular demands in that respect.
Preferably the apparatus has a welding current control arrangement for controlling the welding process. That not only communicates control and reference or target values to the individual units of the apparatus, but it also receives from such units data that it requires for controlling the welding procedure. The welding current control arrangement makes it possible to accurately set and monitor the welding current as well the welding time in order in that way to ensure a good weld for two electrically conductive items.
In a further preferred feature of the invention the welding inverter may be arranged between the welding current control arrangement and the welding transformer.
A further preferred feature of the invention also provides that the welding current control arrangement is connected to the drive. In that case the drive is controlled by the welding current control arrangement so that the welding force and/or the welding electrode position can also be adjusted and monitored by means of the welding current control arrangement. In that respect, especially as the welding current control arrangement has a comprehensive control function, it can also be referred to generally as the welding control. This design configuration permits the three welding parameters listed hereinbefore to be accurately and well matched to each other.
In a further preferred feature of the invention the welding inverter is connected between the welding control and the drive. As an alternative thereto a further separate drive inverter may be connected on the input side of the drive, to connect the drive to the welding control.
Preferably, the one control arm is arranged fixedly and rigidly while the other control arm is arranged to be movable by means of the drive. Additionally or alternatively both control arms may be arranged on a movable actuating or control member. That actuating member is connected to a drive and can be moved thereby. That drive can be supplied with voltage for example by a voltage supply unit which is independent of the above-mentioned three-phase network. As an alternative thereto, the drive may be supplied with voltage from the three-phase network. For that purpose the drive for moving the actuating member is connected to at least one electronic unit of the rectifier circuit and/or the welding inverter. That drive also shares with the welding transformer the welding inverter which is associated therewith. Instead of the welding inverter, a further separate drive inverter may also be connected on the input side of the drive for moving the actuating member. The drive for moving the actuating member may be in the form of an electric motor but alternatively it may also be in the form of a pneumatic or hydraulic drive.
In a further preferred feature of the invention the drive for moving the actuating or control member is connected to the welding control.
The above-mentioned inverters according to the invention preferably have a bridge circuit with transistors as switching members and free-wheeling diodes parallel thereto. Preferably the apparatus uses IGBTs (insulated gate bipolar transistors) in which the free-wheeling diode is integrated with a transistor acting as a switching member in one component.
Preferably, the inverters are arranged on a common component carrier or support, for examples mounting unit or a cooling plate. That can ensure that the number of components required for the inverters, for example feed lines, cooling system and so forth, remains low as those components can be jointly used for a plurality of functions.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of preferred embodiments thereof.